This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the following, a problematic described in the context of focusing and beam forming in the near zone) is presented. However, the present technique can also be used in a wider context of the guiding of electromagnetic waves.
The focusing and collimation (i.e. beam forming) of electromagnetic waves is an established way to increase locally the magnitude of the electric field and, in such a way, to enhance efficiency of sensors, e.g. electro-optical sensors whose operational principles rely on the conversion of the energy propagating in space in the form of an electromagnetic wave into an output voltage or current. The latter sensors (for instance CMOS imaging sensors or photodiodes) are in the heart of the devices produced by Lytro, Raytrix, and Nokia as explained in document U.S. Pat. No. 8,953,064. The same phenomenon of the local field enhancement is used in a variety of other application at different wavelength ranges.
In the optical field, the today level of technologies enables fabrication of highly-integrated components (e.g. chips and optical sensors) with structural elements having nano-scale dimensions, which are close to or even smaller than the wavelength of visible light (see for example the article “A full-color eyewear display using planar wavequides with reflection volume holograms” by H. Mukawa et al., published in the proceedings of SID, vol. 17, no. 3, pp. 185-193, 2009, or the article “Efficient colour splitters for high-pixel density image sensors” by S. Nishiwaki et al., published in Nature Photonics, vol. 7, pp. 240-246, 2013, or in the document US 2014/0111677, or in the article “A 3D integral imaging optical see-through head-mounted display” by H. Hua and B. Javadi, published in the Opt. Express, vol. 22, 13484, 2014). The possibility of manipulating light with the same level of accuracy would become a great breakthrough compared to the state of the art.
The spatial resolution of conventional focusing devices, such as dielectric and metal-dielectric lenses, is limited by the Abbe diffraction limit and typically does not exceed one wavelength in the host media. At the same time, there are many applications which require, or can benefit from, a sub-wavelength resolution (see for example the article “Photonic nanojets”, by A. Heifetz et al., and published in the Journal of Computational Theory Nanoscience, vol. 6, pp. 1979-1992, 2009). This explains the growing interest to the focusing components enabling the sub-wavelength resolution.
Another critical challenge associated with the today mobile and wearable technologies consists in the need for further miniaturization of such devices. The operational principles of the conventional lenses prevent reduction of their dimensions beyond a certain limit (˜10 wavelengths) that constitutes a bottleneck for the future advances in the field. In particular, such a constrain may concern the packaging density of light detectors and may thus handicap further improvement of the image resolution.
Finally, the operational principles of the conventional lenses require a certain index ratio between the lens and host medium materials. The higher the index ratio, the higher the lens focusing power can be achieved. Because of this, in most cases the lenses are separated by air gaps, which require additional space and cause certain difficulties with lens fixation in space and alignment. Fully integrated system can help avoid these problems (see the previous mentioned article “Efficient colour splitters for high-pixel density image sensors”).
However, combination of several dielectric materials with different refractive indexes is rather difficult and not always feasible because of both the technological difficulties and the limited range of the refractive index variation for the optically-transparent materials (typical index value in optical range is n<2). Thus, alternative design concepts are needed.
Nowadays, the most popular focusing elements remain convex dielectric lenses introduced long ago (see FIG. 1 (a)). Such a lens can effectively focus light in a tight focal spot noted FS located on a certain distance noted FL from the lens surface, provided the lens has sufficient aperture size and its profile shape is properly defined with respect to the refractive indexes of the lens material and host medium. The operational principle of the refractive dielectric lenses is based on the Snell's law, which predicts the tilt (refraction) of optical rays at the air-dielectric boundary of the lens due to the different phase velocity in the two media. To enable the desired focusing function, the lens must have the aperture size of at least a few wavelengths in the host medium, with a typical physical size varying from a few microns in case of microlenses to several centimeters in case of camera objective lenses. Their resolution is limited by the Abbe diffraction limit and is typically larger than one wavelength in the host media.
There is also a class of a Fresnel-type diffractive lenses, whose operational principles rely on the interference of the waves diffracted by multiple concentric rings (see FIG. 1 (b)). Compared to refractive lenses (see FIG. 1 (a)), such lenses have smaller thickness, however, they usually suffer from strong chromatic aberrations. Their resolution is limited by the diffraction limit, same as for refractive lenses.
As already mentioned above, the spatial resolution of far-field focusing systems (e.g. refractive and diffractive lenses) is limited by the Abbe diffraction limit set by ˜λ/2n sin α, where λ is the vacuum wavelength, n is the host media refractive index, and is the half aperture angle of the lens. Thus, a higher resolution can be achieved either by increasing the lens aperture size or by reducing the focusing distance. The latter explains the growing interest to nearfield focusing systems. This interest is also strongly supported by the growing number of applications across different domains, which require near-field light processing with the highest possible resolution (see for example the previous mentioned article “Photonic nanolets”).
At present, there are several near-field focusing techniques available, based on subwavelength aperture probes (see the article “Near-field optical microscopy and spectroscopy with pointed probes”, by L. Novotny and S. J. Stranick, published in the Annu. Rev. Phys. Chem. Vol. 57, pp. 303-331, 2006 or the article “Fabrication of optical fiber probes for scanning near-field optical microscopy”, by S. Y. Guo, J. M. LeDue, P. Grütter, and published in mSURJ, vol. 3, no. 1, 2008.), planar subwavelength-patterned structures (see the document U.S. Pat. No. 8,003,965 or the article “Near-field plates: subdiffraction focusing with patterned surfaces” by A. Grbic, L. Jiang and R. Merlin, published in Science, vol. 320, pp. 511-513, 2008), and photonic nanojet microsphere dielectric lenses (see the article “Photonic nanojet-enabled optical data storage” by S.-C. Kong et al., published in Opt. Express, Vol. 16, No. 18, 2008, or the document U.S. Pat. No. 7,394,535, (illustrated in FIG. 1(c)) or the previously mentioned article “Photonic nanolets”). The latter solution (i.e. nanojet microspheres (FIG. 1(c))) is often referred to as the most effective one because microspheres can simultaneously provide the subwavelength resolution and a high level of field intensity enhancement (also noted FIE). As shown on FIG. 1(c), they allow generating a nanojet beam NB.
Despite of the attractive performance characteristics, the use of microspheres is associated with certain difficulties related to their (i) precise positioning, (ii) integration with other optical components, and (iii) non-compatibility with the established planar fabrication techniques. These difficulties affect feasibility and increase the fabrication and assembly costs of the nanojet based devices. Potentially, the assembly problem can be solved using nanoscale patterned structures or hollow tubing (see the document U.S. Pat. No. 8,554,031), but these solutions may not be compatible with some applications.
An alternative solution for nanojet microsphere lenses was proposed based on the solid dielectric cuboids (noted SDC). As demonstrated in the article “Terajets produced by dielectric cuboids” by V. Pacheco-Pena et al., published in Applied Phys. Lett. Vol. 105, 084102, 2014 (and illustrated by FIG. 1(d)), or in the article “Multifrequency focusing and wide angular scanning of terajets” by V. Pacheco-Pena et al., published in Opt. Lett., vol. 40, no. 2, pp. 245-248, 2015, when illuminated by a plane wave, the SDC lenses can also produce condensed beams TB, similar to the nanojet beams observed for microspheres, with subwavelength dimensions, provided the size and shape of cuboids is properly adjusted with respect to the incident wavelength and the refractive index of the cuboid material. According to the previously mentioned article “Terajets produced by dielectric cuboids”, the best spatial resolution (λ/2, where λ is the wavelength in the host medium) and field intensity enhancement (factor of ˜10) is achieved for SDC with dimensions of about one wavelength in the host medium and the refractive index ratio n2/n1˜1.5, where n1 and n2 are refractive indexes of the host medium and cuboid material, respectively.
Although the rectangular shape of SDC lenses can be advantageous for some planar fabrication methods (e.g. micromachining or lithography), the fabrication of SDC lenses operating in the optical range can be difficult or even impossible because of the following constrains:                Strict requirements imposed on the cuboid size and shape;        Absence of materials with the desired refractive indexes (in the optical range, the refractive index of common optical glass and plastics, which can be used as a host medium, varies from n1≈1.3 up to 2.0, whereas, according to the article “Terajets produced by dielectric cuboids”, the desired value of the cuboid lens refractive index should be n2˜2.25 (follows from the suggested ratio n1/n2=1.5 and the refractive index value of a standard glass n1≈1.5) that is out of range for standard optical materials;        No solution provided for setting the position of such lenses in space. This is a critical point because of the miniature size of the cuboids.        
For the completeness of the discussion, it is worth mentioning one more alternative solution for the near-field enhancement available in the optical range. This solution is based on the phenomenon known as surface plasmon polaritons (noted SPP). The SPP phenomenon enables one to create subwavelength hot spots with a very high field intensity.
In particular, SPP-based components find application in color filtering and display technologies (see the article “Plasmonic structures color generation via subwavelength plasmonic nanostructures” by Y. Gu et al., and published in J. Nanoscale, vol. 7, pp. 6409-6419, 2015). However, the SPP fields are tightly coupled to the metal and decay exponentially away from the surface, which prevents the use of SPP devices for the optical systems requiring a “long-range communication” or far-field beam forming. Moreover, the SPP can only be excited under specific conditions that include:                certain material properties of the metals (i.e. negative real part of the relative permittivity that is only intrinsic to some noble metals in the visible light spectrum);        normal E-field component in the incident field; and        use of a SPP launcher (e.g. dielectric prism or grating).        
These constrains are not always acceptable.
Based on the above, it can be concluded that each of the existing focusing methods and components suffers from certain limitations and thus does not fully satisfy the needs of the today and future micro and nanotechnologies. The critical limitations, intrinsic to all (or at least some) of the available focusing devices, are associated with:                physical dimensions of the components,        limited spatial resolution,        limited choice of dielectric materials (limited refractive index variation range),        fabrication/integration difficulties,        certain limitations in the performance characteristics of the devices (e.g. chromatic aberrations and/or polarization sensitive response) linked to the operational principles of these devices.        
Hence, it would hence be desirable to provide a technique enabling control over the field intensity distribution in the near zone, and notably for focusing electromagnetic waves and beam forming in the near zone, which would not suffer from these drawbacks.